Aluminium is a metal used a lot in electricity-conducting structures, because its conductivity is so good. However, aluminium forms an oxide layer on its surface in an air atmosphere, which considerably hampers the conductivity of electricity to or from the aluminium piece. From time to time it is necessary to improve the electrical conductivity of the aluminium piece locally, and this is performed for instance by joining copper pieces to the aluminium piece. Likewise methods are also known where an aluminium and silver join has been made, although not always for reasons of improving conductivity.
When some other material is joined to aluminium, the greatest problem is generally the immediate oxidation of the aluminium in the air atmosphere. The aluminium oxide that is generated is difficult to remove permanently in connection with normal soldering methods. For example commercial cadmium- and fluoride-containing fluxing agents do not remove oxides in sufficient quantities and the join formed by soldering remains porous and weak.
A method is known from WO application 2004/042121, in which a silver coating layer is formed on an aluminium electrode support bar. The contact between the aluminium and the coating material is achieved in particular with a thermal spray coating method. The thermal spraying technique breaks the passivation layer of the aluminium i.e. the oxide layer, so that the contact of the metals is good enough for a metallurgical join to form and for the coating to attach to its substrate.
A hermetic coating on the surface of the aluminium can be achieved with thermal spraying methods, but the equipment required by the methods is rather expensive as yet. In addition, typically in thermal spraying methods, not all of the coating material ends up on the surface of the piece to be coated, and instead, some of the coating material is wasted with regard to the efficiency of the method.
EP patent publication 28763 describes a method for joining metal pieces to each other. The pieces may be the same metal or different metals. The metal joins described in the patent are Al—Al, Cu—Cu and Al—Cu, and also described are joins in which an intermediate agent is introduced between the pieces to be joined, such as an insert of silicon, aluminium-silicon alloy or silver. Bonding takes place by means of pressure at raised oxygen pressure, exploiting the eutectic reaction occurring between the metals. The temperature required by the eutectic reaction depends on the materials to be bonded and the temperature used is in the region of the eutectic temperature—+50° C. The description of the method reveals that when an oxygen-enriched atmosphere is used for heating the pieces, the oxide layers formed on the contact points of the pieces are squeezed out with the liquefied eutectic alloy. Pure oxygen is used in the examples and the pressure used was in the region of 150-710 bar.
The bonding of the pieces to each other described in the EP publication takes place at very high pressure, which squeezes the impurities and the layer that was oxidized during heating out of the joining point. However, the use of an oxygen atmosphere in heating and the high pressure make this a very expensive bonding method.
JP application 57195592 relates to a method to join silver and aluminium to each other, in which the oxidation of the surfaces is prevented by making a join by hot pressing and in a vacuum or inert atmosphere.
The metal join described in the JP application by means of hot pressing and in a vacuum or inert atmosphere is not a particularly cost-effective join solution.